Okay -- this morning I left the house for my multi-mode commute, and as I rolled the bike out to the car, I noted that my rear tire was flat. I had ridden yesterday, so it seemed odd, but I was late, so I decided I'd fix it at my 'mid' point.

I checked the tire and rim for sharps/burrs/thorns, etc... found nothing. I found the hole in the tube, patched it, re-mounted and tried re-inflating. This resulted in another flat, though I thought I might have missed the actual hole in my haste. I re-patched the tube, remounted, and all seemed well enough... a mile later, I was flat again. I re-checked everything, straightened out some sloppy spots on the rim tape, installed a new tube, and started again. Another mile/mile and a half, and another flat. I limped to the office by re-inflating and riding very gently.... I'm now in my office with the tire and wheel in hand, looking furiously for a thorn or something in the tire, or some sort of burr or sharp on the wheel, but nothing.

What am I missing?